In a multipoint conference service, voice data of each participant is transmitted to a multipoint conference server. The multipoint conference server transmits to each participant a voice signal that is a mixed voice signal of all other participants. FIG. 4 is a drawing showing a typical configuration example with regard to a voice mixing device for a multipoint conference (refer to Patent Documents 1 and 2). In FIG. 4, with the number of participants in the conference as M (M being a positive integer greater than or equal to 2), a voice signal for a 1st, 2nd, . . . M-th speaker is inputted from respective voice input terminals 501, 502, . . . 50M. The respective input voice signals are inputted to a full adder 520 and adders 531, 532, . . . 53M, respectively. The full adder 520 outputs a voice signal, obtained by adding all input voice signals, to the adders 531, 532, . . . 53M. The adders 531, 532, . . . 53M subtract voice signals respectively inputted thereto from the voice signal inputted from the full adder 520, and output the subtracted voice signals to voice output terminals 571, 572, . . . 57M, respectively. According to this type of voice mixing device, it is possible to mix and listen to voice signals of people other than one's self.
However, in the multipoint conference service, particularly in cases in which participants are using mobile telephones, background noise is often superimposed on the voice signals. In such types of mobile telephones and the like, noise suppression processing is effective. As an example of noise suppression processing, a method is known in which, after performing frequency domain analysis with respect to input voice by Fourier transform or the like, superimposed noise is estimated, and a noise component is subtracted from the input voice (for example, refer to non-Patent Document 1).
[Patent Document 1]
JP Patent Kokai Publication No. JP-P2005-269347A
[Patent Document 2]
JP Patent Kokai Publication No. JP-A-10-75310
[Non-Patent Document 1]
Yamato Kazuhiro, Sugiyama Akihiko, Kato Masanori, Post-Processing Noise Suppressor with Adaptive Gain-Flooring for Cell-Phone Handsets and IC Recorders, International Conference on Consumer Electronics (ICCE), 2007, pp. 1-2.